用a代替
x=4t
y=-3t
则r=√(x²-y²)=5|t|
sina=y/r=-3t/5|t|
cosa=x/r=4t/5|t|
所以t>0,sina=-3/5,cpsa=4/5
t<0,sina=3/5,cpsa=-4/5
所以
t>0,原式=-2/5
t<0,原式=2/5